Field of the Invention
The invention relates to a solar cell stack.
Description of the Background Art
A solar cell stack having multiple solar cells is known from US Publication 2006/0021565. In a first embodiment therein, a thin heteroepitaxial semiconductor seed layer is applied to an active Si substrate, and then the additional III/V solar cells are epitaxially grown on the seed layer. In another embodiment, a III/V multi-junction solar cell that has already been grown, consisting of a stack of multiple III/V semiconductor solar cells, is bonded as the first solar subcell to a silicon solar cell with an n+ doped surface, which is to say joined, in order to form a covalent, low resistance connection. The first solar subcell here has a tunnel diode with two differently doped layers of GaAs at the surface that is to be bonded. Both the silicon surface and the surface of the III/V solar subcell are made extremely planar. A difficulty in the joining consists in great bowing of the bonded layers as a result of the different crystal lattices and different coefficients of thermal expansion. An attempt is made to reduce the bowing of the solar cell stack by means of a special backside coating.
A III/V multi-junction solar cell with a metamorphic intermediate layer is known from “Current-matched Triple Junction Solar Cell Reaching 41.1% Conversion Efficiency Under Concentrated Sunlight”, by Wolfgang Guter et al., in Applied Physics Letters 94, 223504 (2009). Such metamorphic intermediate layers act as an intermediary between the different lattice constants of stacked semiconductor solar cells. The different lattice constants of the semiconductor layers are a result of the choice of the semiconductor materials based on different band gaps in order to increase the efficiency of the solar cell stack, wherein the metamorphic intermediate layers permit crystalline growth of a second semiconductor solar cell on a first semiconductor solar cell with a different lattice constant. It is a disadvantage of the metamorphic intermediate layers that, even though the surface of the second semiconductor solar cell is crystalline in structure, because of misfit dislocations and other crystal defects, it has a large number of mesa-like elevations and generally an increased roughness, and hence the surface of the second semiconductor solar cell does not appear suitable for wafer bonding. Moreover, after epitaxy, the solar cells are more strongly bowed as a result of the strain on the lattice-mismatched layers, which likewise precludes wafer bonding. For this reason, exclusively epitaxial process steps are used for producing multi-junction solar cells such as, in particular, a triple junction cell. This makes it possible to avoid a resource-intensive semiconductor bonding process.
Furthermore, additional III/V multi-junction solar cells are known from “Development of Advanced Space Solar Cells at Spectrolab,” by J. Boisvert et al, in Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE, 20-25 Jun. 2010, Honolulu, ISSN: 0160-8371. In one embodiment, solar cell stacks of more than three individual semiconductor solar cells are produced by semiconductor bonding (SBT) of a first solar subcell to a second solar subcell. In order to obtain a sufficiently planar surface, low surface roughness, and low bowing for bonding, the first solar subcell and the second solar subcell are epitaxially grown on the relevant substrates in a lattice-matched fashion, which is to say without metamorphic intermediate layers. In this process, one of the solar subcells is grown on the very costly InP substrate. In production, such expensive substrates are generally separated either before joining or after joining and re-used.
Additional manufacturing processes and embodiments for a III/V triple junction solar cell stack are known from “Metamorphic GaAsP buffers for Growth of Wide-bandgap InGaP Solar Cells”, by J. Simon et al, in Journal of Applied Physics 109, 013708-1 to 013708-5, (2011).
In addition, a joining of a first solar cell stack to a second solar cell stack is known from US 2010/011 6327 A1, wherein one of the two solar cell stacks has a metamorphic intermediate layer. Each of the two solar cell stacks has a bonding layer made of InGaAs.